<asp:Button runat="server" ID="btnUserDelete" Text="Delete" CssClass="GreenLightButton"
                           OnClick="BtnUserDelete_Click"
                           OnClientClick="return UserDeleteConfirmation();" 
 meta:resourcekey="BtnUserDeleteResource1" />

Ich habe versucht:

function UserDeleteConfirmation() {
        if (confirm("Are you sure you want to delete this user?"))
            return true;
        else
            return false;
}

und

function UserDeleteConfirmation() {
    if (confirm("Are you sure you want to delete this user?")) {
            __doPostBack(btnUserDelete, '');
    }

    return false;
 }

Und keiner von ihnen funktioniert.